An AGC voltage generator is devised for use in combination with a variable gain amplifier. The amplification factor of such an amplifier can be changed with the application of a control voltage thereto. The AGC voltage generator generates the control voltage to adapt the amplifier gain to the magnitude of the input signal level. AGC, or automatic gain control, circuits have been utilized for many years. However, the well known AGC circuits generally do not include any substantial holding time, i.e. the time during which the AGC voltage is applied to the amplifier after the input signal causing the AGC voltage has been removed. A holding time is desirable in many applications where the input signal is a periodic signal and it is desirable to maintain a relatively low sensitivity between periodic signals. For example, in conjunction with a radar receiver, it may be desirable to maintain a reduced sensitivity after each return from a target so that sidelobe interference is minimized.
Prior art AGC circuits utilize a peak detector and capacitor storage to drive a variable gain amplifier. These circuits are unsatisfactory because the holding time is severely limited by capacitor value and the inherent leakage of the stored charge, which will result in a relatively rapidly decaying control voltage level during the holding period.
On U.S. Pat. No. 3,579,138, entitled "Automatic Gain Presetting Circuit," issued May 18, 1971, illustrates a circuit including a shift register for providing an incremental AGC voltage. This circuit is designed strictly for presetting the gain of an amplifier and does not teach the generation of a continously variable AGC voltage, an AGC circuit with a holding timer (rather than a preset), or an AGC circuit capable of compensating for certain non-linearities in the amplifier transfer function and in the AGC control characteristics.